In conventional casting, the metal is superheated above its liquidus temperature (i.e. the liquidus being the temperature above which the alloy is completely liquid). A minimum superheat is required to ensure that the metal does not solidify prematurely, particularly when molding thin-walled molded articles. Superheating metals which are prone to oxidation has attendant process control challenges to provide and maintain an inert atmosphere.
Articles which are cast from superheated melts often are not sound in that shrinkage porosity and entrapped gases are not uncommon. In addition, their mechanical properties such as tensile strength, yield stress, and elongation suffer, and this is attributed to a microstructure characterized by coarse grains and dendrites.
These problems have been recognized and extensive work has been done to find other ways of processing metal alloys to improve the mechanical properties of cast articles. In particular, through the use of well known semi-solid metal processing techniques molded articles may be produced with much higher mechanical properties as a result of the generation of a favorable alloy microstructure and by reductions in alloy porosity. Moreover, semi-solid processing techniques provide further advantages in that the relatively low temperature of the alloy slurry provides for a longer useful life of the mold than the die-casting method (e.g. lower thermal shock, and reduced amount of liquid-metal corrosion caused by processing fully molten metals), and improved molding accuracy of the molded article. Common semi-solid processing techniques include semi-solid injection molding, rheocasting, and thixoforming.
Semi-solid injection molding (SSIM) is a metals-processing technique that utilizes a single machine for injecting alloys in a semi-solid state into a mold to form an article of nearly net (final) shape. SSIM involves the steps of partial melting of an alloy material by the controlled heating thereof to a temperature between the liquidus and the solidus (i.e. the solidus being the temperature below which the alloy is completely solid) and then injecting the slurry into a molding cavity of an injection mold. SSIM avoids the formation of dendritic features in the microstructure of the molded alloy, which are generally believed to be detrimental to the mechanical properties of the molded article. The structure and steps of SSIM are described in more detail with reference to the description of the preferred embodiment of the present invention provided hereinafter and with reference to U.S. Pat. No. 6,494,703, the disclosure of which is herein incorporated by reference.
By contrast, rheocasting refers to a process of manufacturing billets or molded articles through casting or forging semi-solid metallic slurries having a predetermined viscosity. In conventional rheocasting, molten alloy is cooled from a superheated state and stirred at temperatures below the liquidus to convert dendritic structures into spherical particles suitable for rheocasting, for example, by mechanical stirring, electromagnetic stirring, gas bubbling, low-frequency, high-frequency, or electromagnetic wave vibration, electrical shock agitation, etc.
Thixocasting refers to a process involving reheating billets manufactured through rheocasting back into a metal slurry and casting or forging it to manufacture final molded articles.
For instance, U.S. Pat. No. 5,901,778 describes an improved rheocasting method and extruder apparatus for producing a semi-solid metal alloy slurry having a solids content between 1 and 50% that is characterized by structure and steps whereby molten metallic alloy material is introduced into an agitation chamber, that is heated about 100 degree C higher than a liquidus temperature of the molten metallic material, wherein the alloy is cooled and agitated by a cooled screw-shaped stirring rod, having a temperature below a temperature of the semi-solid, to produce the semi-solid slurry.
U.S. patent application Ser. No. 2004/0173337 describes an improved rheocasting method and apparatus for producing a non-dendritic, semi-solid metal alloy slurry having a solids content of about 10% to about 65% that is characterized by structure and steps whereby problems associated with accumulation and removal of metal from surfaces of the apparatus contacting the slurry are reduced or eliminated.
U.S. patent application Ser. No. 2004/0055726 describes a rheocasting method and apparatus for die casting molded articles that is characterized by structure and steps for applying an electromagnetic field to stir a molten metal as it is being loaded into a slurry forming portion of a shot sleeve whereby the slurry is stirred until cooled below its liquidus temperature prior to its transfer to a casting portion of the shot sleeve. Preferably, the stirring is maintained until the slurry achieves a solid fraction in the range of 0.1 to 40%, alternatively the slurry is stirred until the solid fraction is in the range of 10 to 70%. Related U.S. patent applications Ser. Nos. 2004/0055727, 2004/0055734, and 2004/0055735 describe similar structure and steps for manufacturing billets for thixocasting, manufacturing metallic materials for rheocasting or thixoforming, and for manufacturing a semi-solid metallic slurry, respectively.
U.S. Pat. No. 6,311,759 describes a process for producing a feedstock billet material that is characterized in that it is produced from a melt at substantially its liquidus temperature whereby a microstructure of the feedstock is rendered especially suitable for subsequent thixocasting in the semi-solid range of 60 to 80% primary solids. This patent is significant in that it recognizes that metal alloys cast from at a near liquidus temperature will result in a favorable grain structure characterized by primary grains that are equi-axed and globular with no dendrites.
The process of SSIM is however generally preferred as it provides for several important advantages relative to the other semi-solid processing techniques. The benefits of SSIM include an increased design flexibility of the final article, a low-porosity article as molded (i.e., without subsequent heat treatment), a uniform article microstructure, and articles with mechanical and surface-finish properties that are superior to those made by conventional casting. Also, because the entire process takes place in one machine and in an ambient environment of inert gas (e.g., argon), alloy evaporation and oxidation can be nearly eliminated. The SSIM process also provides for energy savings in that it does not require the heating of the alloy above its liquidus temperature.
Although a 5-60% solids content is generally understood to be the working range for SSIM, it is also generally understood that practical guidelines recommend a range of 5-10% solids for injection molding thin-walled articles (i.e., articles with fine features) and 25-30% for articles with thick walls. The foregoing is described in U.S. Pat. No. 5,040,589.
Notwithstanding the foregoing, a recently published discovery by the inventor of the present invention has shown that the range of percentage of solids in SSIM processing can be advantageously extended into an ultra-high solids range between 60 and 85%. The foregoing ultra-high solids process is fully described in commonly assigned U.S. patent application Ser. No. 2003/0230392.
The lower limit of 5% solids fraction has been sustained by those skilled in the art because of a belief that to lower the solids fraction any further would obviate any advantages achieved by semi-sold processing. In particular, with a low or non-existent solids content, the fluidity of the alloy is expected to increase, resulting in an increase in turbulence in the flow front thereof as the molding cavity is being filled, and thereby increasing the likelihood of porosity and entrapped gases in the final article.
Notwithstanding the foregoing, it is known to configure structure and steps for SSIM processing with a percentage of solids as low as 2% under certain conditions.
For instance, U.S. Pat. No. 5,979,535 describes a method for injection molding a molded article having both lower and higher solid fraction portions therein, the method characterized in that structure and steps are provided for establishing a temperature distribution in the semi-molten slurry in the direction of injection, by the controlled heating thereof in an extruder cylinder, whereby the slurry contemporaneously includes a low and a high solids fraction portions for sequential injection into the molding cavity. In a cited example, an orifice holder is molded in which a high strength head portion is formed from a melt portion having about 2% solids whereas a more accurately molded threaded portion is formed from a melt portion having about 10% solids.
However, the molding of thin-walled molded articles, particularly those having a thickness below 2 mm, using SSIM at typical low levels of solids fraction (i.e. 5%) can be problematic because of premature alloy solidification that results from the reduced fluidity of the alloy metal, relative to die casting, and because of the high thermal conductivity of typical molding alloys (e.g. Magnesium alloy AZ91D).
U.S. Pat. No. 6,619,370 is directed at solving the problems of molding thin-walled molded articles using SSIM. In particular, structure and steps are provided for increasing the fluidity of the semi-molten melt and for providing increased degassing of the molding cavity. It is stated therein that the solid fraction of the semi-molten metal slurry must be set within a range exceeding 3% and below 40% to avoid excessive warping of the thin-walled molded article.
However, it remains a challenge to produce thin-walled molded articles using SSIM without resort to significant overheating of the alloy above the liquidus temperature and the resulting reduction in mechanical properties.
Accordingly, an advantage of the present invention is that an injection molding process is provided for producing thin-walled metal articles with improved structural integrity and superior mechanical properties relative to those produced by traditional casting methods.